1. Field of the Invention
The present invention relates generally to protective devices for communications systems, and more particularly, to electronic communication-line protectors for protection of associated communication equipment such as exchange servicing modules against the occurrence of abnormal or surge current as possibly induced by accidental shorts of power transmission lines.
2. Description of Related Art
Conventionally, protective devices for use in protecting associative equipment in communications systems are known, one of which has been disclosed, for example, in Published Unexamined Japanese Patent Application (PUJPA) No. 3-5135. A configuration of the prior known protective device is shown in FIG. 8, wherein this device is generally designated by numeral 40. The protective device 40 includes a pair of gas tube arresters 41a, 41b, which are interconnected in series between two lines A, B coupled to input terminals which are linked to communication lines. The series combination of gas tube arresters 41a, 41b has a common node coupled to ground. The protective device 40 also includes a series combination of variable resistors or "varistors" 43a, 43b connected between the lines A, B at the output terminals, which are in turn connected to target equipment being protected. The series combination of varistors 43 has a common node grounded, and is parallel with the series combination of gas tube arresters 41a and 41b. Provided between these series combinations 41, 43 are a pair of a positive thermally sensitive resistor or "thermistor" 45a and a thick-film resistive element 47a inserted in series in a first line A, and another pair of a positive thermistor 45b and its associated thick-film resistor 47b in the other line B as shown.
As generally known in the art, the positive thermistors 45a, 45b suffer from a problem in that possible variations in resistance are as large as about 20 percent (%). In this case, in order to decrease the amount of impedance imbalance between the lines A, B, the thick-film resistors 47a, 47b are trimmed in shape to ensure that the resulting combined resistance value of the positive thermistors 45 and thick-film resistors 47 falls within a limited range of accuracy of approximately 1%.
With the communication-line protective device 40, when surge current enters or "invades" from the communication-line linkage input terminals due to accidental electrical shorts between adjacent ones of the power feed lines, the positive thermistors 45a, 45b self-heat and rapidly increase the resistance values thereof, thereby attempting to provide protection for the associative equipment being protected by suppressing or eliminating the applied surge current.
Unfortunately, the prior known protective device 40 has encountered several problems. The surge current protection scheme does not come without accompanying penalties: first, the quality of voice information or data under communication may be degraded; second, the circuit board is increased in area needed for mounting the positive thermistors 45a, 45b thereon.
More specifically, the positive thermistors 45 exhibit a resistance-to-temperature characteristic that is relatively large in the variation ratio of resistance to ambient temperature even at temperatures below the Curie temperature, with its deviations being increased. This serves to adversely affect the impedance imbalance amount as follows. Even in the case where the combined resistance value of the positive thermistors 45 and thick-film resistors 47 can be at one time optimized in accuracy, if ambient temperature varies, a difference may arise between the resulting resistance values of the first thermistor/resistor pair (45a, 47a) associated with the first line A and of the second thermistor/resistor pair (45b, 47b) with second line B, due to the occurrence of deviations in the temperature characteristics of the respective thermistors 45a, 45b. Such an increase in impedance imbalance between the lines A, B causes degradation in voice quality at exchange servicing modules such as private branch exchanges (PBXs).
In addition, it should be strictly required that the positive thermistors 45a, 45b be relatively large in size in order to ensure that they may withstand the application of surge current or eliminate the occurrence of any firing even under destruction in the worst case. Obviously, this causes the associative printed circuit board on which the thermistors 45 are mounted to increase in area accordingly.